Accuracy of Aquaria Pharm's Nitrate Test

No live plants. Only trace amounts of algae. Nothing that would "eat"
nitrates. Yet with two more fish my AP Nitrate tester has read a
suspiciouly low nitrate number of 5ppm for the entire week where it usually
would increase from 5ppm to 10ppm nitrates with less fish. I did add an
additional penguin 170 filter to my tank at the beginning of this week, but
it is brand new and shouldn't help in any significant way with the
bio-filtering and wouldn't "eat" nitrates anyways. I'm certain that my
biofilter did not crash. I considered that possibilty when I added the fish
and I tested daily for ammonia. I got all zero ammonia test results. I
didn't test for nitrites because I ran out last month and was too lazy to
get another nitrite test kit.

I've had this AP nitrate tester for about a year.

So... am I paranoid or have other folks gotten suspicious readings using
Aquaia Pharm's Nitrate tester?

Thanks,
E.Otter